I just put in a paid class, and the number of paid slots doesn’t come up until you’ve put the $$ in. Before I put the $, there’s only Free spaces. Once I entered the $, then it shows how many paid slots you want.
One reason I know that this is the way it works is that I was copying a previous event, which was also paid. You have to put in the cost each time, and I didn’t have a space for Paid attendees until I’d put in the cost.